Supportive relationships, especially those with primary caregivers, are crucial for both the physical survival and the healthy social-emotional development of infants and toddlers. This guide, the first in a series on infant mental health from the Center for Program Excellence, describes how supportive relationships promote the social-emotional development of very young children. The guide is written for infant/family program leaders and contains exercises that focus on supervisory issues and on strategies that can be used in working directly with families. The report is presented in the following sections: (1) "What Is Infant Mental Health?" defining the concept of infant mental health; (2) "What's So Important about Good Relationships?" discussing the role of relationships in promoting and supporting children's mental health; (3) "Good Relationships Are Catching," highlighting how strong relationships between supervisors and staff as well as staff and families foster nurturing parent-child relationships and promote infant mental health; (4) "Practical Strategies for Promoting Infant Mental Health," identifying specific steps leaders can take to promote infant mental health at the supervisory and organizational levels; and (5) "Guidelines for Identifying Infant Mental Health Needs," discussing key steps in identifying children who may have specific mental health needs or who may require additional support in developing social-emotional skills. The guide's two appendices present leadership dilemma individual or group activities and brainstorming activities regarding key qualities in building strong relationships. (Contains 25 references and lists 16 additional resources.) (KB)